In a vertical power device such as a vertical MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) and an IGBT (Insulated Gate Bipolar Transistor), an applied voltage is maintained by extending a depletion layer in the vertical direction of a drift layer. In such a device, in a termination region located on an outer periphery of a cell region through which an electric current flows, a depletion layer also extends laterally. Thus, an electric field is generated on the surface of the termination region. At this time, if an electric field distribution of the termination region is changed by an influence from outside the device, it causes reliability deterioration such as fluctuation in a breakdown voltage of the device or a leak current.
In order to ensure reliability of the vertical power device, the electric field distribution on the surface of the termination region needs to be optimized. If the electric field on the surface is high, impact ionization occurs, and a generated hot carrier jumps into a field insulating film. The electric field distribution of the termination region is changed by a charge of the carrier which has jumped in, and reliability is deteriorated.
In order to ensure the termination breakdown voltage, a structure in which a guard ring (hereinafter also referred to as GR) layer is formed on the surface of the termination region is proposed (See JP-A 2000-277726 (Kokai), for example). Also, a structure in which an embedded GR layer is formed in a drift layer of the termination region is proposed (See JP-A 2009-88345, for example).
In the structure in which the GR layer is formed, the electric field distribution can be changed by a design of the number of GRs and an interval between the GRs. However, if the electric field on the surface is to be reduced, the number of GRs needs to be increased, which enlarges the termination region. In a device of a limited size, a problem is caused that the enlarged termination region reduces an effective area in the device through which an electric current flows.